Knocking control system for internal combustion engines

ABSTRACT

A knocking control system for an internal combustion engine with inlet valves and exhaust valves having valve timing thereof controlled depending on operating conditions of the engine. It is determined whether or not knocking has occurred in the engine on the basis of a detection parameter indicative of knocking and at least one discrimination parameter. The operation of the engine is controlled by the use of at least one control parameter in response to the determination result, so as to eleminate knocking. The values of the discrimination and control parameters are selected in accordance with the actual valve timing. When abnormality in the valve timing control is detected, the valve timing is held at predetermined valve timing irrespective of engine operating conditions, and values of the at least one discriminating parameter and the at least one control parameter are selected, which correspond to the held predetermined valve timing. Alternatively, when abnormality in the valve timing control is detected, the above control of the engine operation is inhibited.

BACKGROUND OF THE INVENTION

This invention relates to a knocking control system for internalcombustion engines, and particularly to a system of this kind for aninternal combustion engine equipped with a device which varies theopening and closing timing or lift of inlet valves and exhaust valves ofthe engine depending on operating conditions of the engine in order toprevent knocking.

An internal combustion engine is conventionally known, e.g. fromJapanese Patent Publication (Kokoku) No. 49-33289, which is capable ofchanging the valve timing (valve opening period or valve lift) of inletvalves and/or exhaust valves between low speed valve timing suitable fora lower engine rotational speed region and high speed valve timingsuitable for a higher engine rotational speed region, in order toenhance the charging efficiency or combustion efficiency.

Further, a control method for internal combustion engines is known, e.g.from Japanese Patent Publication (Kokoku) 57-30980, in which knockingoccurring in the engine is detected, and the ignition timing is retardedupon detection of knocking to thereby eliminate the knocking.

However, if knocking control is effected in an internal combustionengine equipped with the valve timing device, there occur the followingproblems:

Knocking is caused by abnormal combustion within engine cylinders. Themagnitude (knocking noise level) and frequency of knocking, as well asthe crank angle at which knocking occurs vary if the valve timing and/orlift of the inlet and exhaust valves and/or the compression ratio vary.Further, the noise level (background level) of the output of a knockingsensor, on the basis of which a knocking discrimination level is set,varies at the time of changeover of the valve timing when changeover ofrocker arms and oil passages, ete. is effected. Also, the ignitiontiming at which knocking occurs and the optimal ignition timing (MBT)are different between the low speed valve timing and the high speedvalve timing. Therefore, knocking is not eliminated to thereby causedamage to the engine and degraded driveability and hence marketabilityof same, unless the retarding amount, retarding speed, advancing amount,and advancing speed of the ignition timing are set to respectiveappropriate values depending on the actual valve timing when theignition timing is retarded upon detection of knocking to eliminate sameand when it is advanced after elimination of the knocking.

In order to solve the above problems, if the values of discriminationparameters used by knocking discriminating means or the values ofcontrol parameters used by knocking control means are varied inaccordance with changeover of the valve timing responsive to engineoperating conditions, knocking control can be properly carried outirrespective of the actual valve timing. However, if a failure occurs inthe valve timing control means in such a case, the values of thediscrimination parameters or the control parameter cannot be variedcorrectly in accordance with changeover of the value timing, resultingin occurrence of knocking.

SUMMARY OF THE INVENTION

It is the object of the invention to provide a knocking control systemfor an internal combustion engine, which is capable of properlyeffecting knockign control even if a failure has occurred in valvetiming control means.

To attain the above object, the present invention provides a knockingcontrol system for an internal combustion engine having inlet valves andexhaust valves, at least one of the inlet valves and the exhaust valveshaving valve timing thereof controlled by valve timing control meansdepending on operating conditions of the engine.

According to a first aspect of the invention, the knocking controlsystem is characterized by comprising:

valve timing detecting means for detecting the valve timing controlledby the valve timing control means;

knocking parameter detecting means for detecting a detection parameterindicative of knocking occurring in the engine;

knocking discriminating means for determining whether or not knockinghas occurred in the engine on the basis of the detection parameterdetected by the knocking parameter means and at least one discriminationparameter;

knocking control means responsive to an output from the knockingdiscriminating means indicative of a determination result that knockinghas occurred, for controlling an operation of the engine by the use ofat least one control parameter, so as to eliminate knocking;

parameter value selecting means for selecting a value of at least one ofthe at least one discrimination parameter and at at least one controlparameter, which corresponds to the valve timing detected by the valvetiming detecting means;

abnormality detecting means for detecting abnormality in the valvetiming control means; and

failsafe means for holding the valve timing at predetermined valvetiming irrespective of an operating condition in which the engine isoperating, and selecting a value of at least one of the at least onediscrimination parameter and the at least one control parameter, whichcorrespond to the held predetermined valve timing, when abnormality inthe valve timing control means is detected by the abnormality detectingmeans.

According to a second aspect of the invention, the knocking controlsystem is characterized by comprising:

knocking discrimination means for determining whether or not knockinghas occurred in the engine;

knocking control means responsive to the valve timing controlled by thevalve timing control means and an output from the knockingdiscriminating means, for controlling an operation of the engine so asto eliminate knocking;

abnormality detecting means for detecting abnormality in the valvetiming control means; and

failsafe means for inhibiting operation of the knocking control meanswhen abnormality in the valve timing control means is detected by theabnormality detecting means.

The above and othe objects, features, and advantages of the inventionwill become more apparent from the ensuing detailed description taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the whole arrangement of a fuelsupply control system for an internal combustion engine incorporating aknocking control system according to the invention;

FIG. 2 is a longitudinal cross-sectional view of essential parts of theengine;

FIG. 3 is a transverse cross-sectional view of a connection-changeovermechanism;

FIGS. 4, 4A and 4B are schematic diagrams showing an oil-feeding systemand an oil pressure-changeover device;

FIG. 5 is a diagram showing set basic fuel injection periodcharacteristics for low speed valve timing and high speed valve timing;

FIG. 6 is an enlarged view of a portion encircled in FIG. 5;

FIG. 7 is a diagram showing a T_(VT) table;

FIG. 8 is a diagram showing lower speed and higher speed valve timingregions;

FIGS. 9, 9A, 9B, and 9C are flowcharts of a program for controlling thechangeover of the valve timing;

FIG. 10 is flowchart of a subroutine for obtaining basic fuel injectionperiod values Ti_(L) and Ti_(H) from respective Ti_(L) and Ti_(H) maps;

FIG. 11 is a flowchart of a subroutine for calculating a value T_(VT) ;

FIG. 12 is a flowchart of a program for controlling ignition timing whchis executed in the knocking control system of the invention;

(A) and (B) of FIG. 13 are block diagrams illustrating an arrangementfor determining occurrence of knocking;

FIG. 14 is a flowchart of a program for controlling knocking accordingto a first aspect of the invention, which is executed in the knockingcontrol system of the invention;

FIG. 14A is a flowchart of a program for controlling knocking accordingto a second aspect of the invention;

FIG. 15 is a flowchart of a subroutine corresponding to a step 1406 ofthe program of FIG. 14;

FIG. 16 is a flowchart of a subroutine corresponding to a step 1407 ofthe program of FIG. 14;

FIG. 17A is a graph showing a multiplier term and FIG. 17B is a graphshowing an addend term to be used for calculation of a knockingdiscrimination level executed at a step 1404 in FIG. 14;

FIG. 18 is a graph showing the knocking discrimination level calculatedat the step 1404 in FIG. 14;

FIG. 19 is a graph showing output from a knocking sensor appearing inFIG. 1;

FIG. 20 is a flowchart of a subroutine for fail-safe operation to becarried out at a step 1409 in FIG. 14;

FIG. 21 is a flowchart of a subroutine for fail-safe operation to becarried out at a step 1409' in FIG. 14A; and

FIG. 22 is a graph showing a predetermined value θ_(RETF/S) of acorrection variable θig_(KN) appearing in FIG. 21.

DETAILED DESCRIPTION

The knocking control system according to the invention will now bedescribed in detail with reference to the drawings showing an embodimentthereof.

Referring first to FIG. 1, there is shown the whole arrangement of afuel supply control system for an internal combustion engineincorporating the knocking control system according to the invention. Inthe figure, reference numeral 1 designates an internal combustion engineof DOHC in-line 4 cylinder type for automotive vehicles, in which twopairs of inlet and exhaust valves are provided for each cylinder.Connected to the cylinder block of the engine 1 is an intake pipe 2across which is arranged a throttle body 3 accommodating a throttlevalve 3' therein. A throttle valve opening (θ_(TH)) sensor 4 isconnected to the throttle valve 3' for generating an electric signalindicative of the sensed throttle valve opening and supplying same to anelectronic control unit (hereinafter called "the ECU") 5.

Fuel injection valves 6, only one of which is shown, are inserted intothe interior of the intake pipe at locations intermediate between thecylinder block of the engine 1 and the throttle valve 3' and slightlyupstream of respective intake valves, not shown. The fuel injectionvalves 6 are connected to a fuel pump, not shown, and electricallyconnected to the ECU 5 to have their valve opening periods controlled bysignals therefrom.

Ignition plugs 22 provided for respective cylinders of the engine 1 areconnected via a driving circuit 21 to the ECU 5 which controls theignition timing θig of the ignition plugs 22.

Further, an electromagnetic valve 23 for valve timing changeovercontrol, described hereinafter, is connected to the output of the ECU 5which controls opening and closing of the electromagnetic valve 23.

On the other hand, an intake pipe absolute pressure (P_(BA)) sensor 8 isprovided in communication with the interior of the intake pipe 2 at alocation immediately downstream of the throttle valve 3' for supplyingan electric signal indicative of the sensed absolute pressure within theintake pipe 2 to the ECU 5. An intake air temperature (T_(A)) sensor 9is inserted into the intake pipe 2 at a location downstream of theintake pipe absolute pressure sensor 8 for supplying an electric signalindicative of the sensed intake air temperature T_(A) to the ECU 5.

An engine coolant temperature (T_(W)) sensor 10, which may be formed ofa thermistor or the like, is mounted in the cylinder block of the engine1, for supplying an electric signal indicative of the sensed enginecoolant temperature T_(W) to the ECU 5. An engine rotational speed (Ne)sensor 11 and a cylinder-discriminating (CYL) sensor 12 are arranged infacing relation to a camshaft or a crankshaft, not shown, of theengine 1. The engine rotational speed sensor 11 generates a pulse as aTDC signal pulse at each of predetermined crank angles whenever thecrankshaft rotates through 180 degrees, while thecylinder-discriminating sensor 12 generates a pulse at a predeterminedcrank angle of a particular cylinder of the engine, both of the pulsesbeing supplied to the ECU 5. A knocking sensor 24 is mounted in aperipheral wall of the cylinder block of the engine 1 at a location inthe vicinity of the top-dead-center of an engine cylinder, for sensingvibration of the engine and supplying an electric signal indicative ofthe sensed vibration to the ECU 5. The knocking sensor 24 is adapted toresonate with the frequency of knocking taking place within thecylinder. Such knocking sensors may be provided for respective cylinderto detect knocking more accurately.

A three-way catalyst 14 is arranged within an exhaust pipe 13 connectedto the cylinder block of the engine 1 for purifyng noxious componentssuch as HC, CO, and NOx. An O₂ sensor 15 as an exhaust gas ingredientconcentration sensor is mounted in the exhaust pipe 13 at a locationupstream of the three-way catalyst 14, or sensing the concentration ofoxygen present in exhaust gases emitted from the engine 1 and supplyingan electrical signal indicative of the sensed oxygen concentration tothe ECU 5.

Further electrically connected to the ECU 5 are a vehicle speed sensor16, a gear position sensor 17 for detecting the shift lever position ofa transmission, and an oil pressure sensor 18 for detecting oil pressurein oil feeding passages (88a, 88e in FIG. 2), referenced to hereinafter,of the engine 1. Signals from these sensors are supplied to the ECU 5.

The ECU 5 comprises an input circuit 5a having the functions of shapingthe waveforms of input signals from various sensors, shifting thevoltage levels of sensor output signals to a predetermined level,converting analog signals from analog-output sensors to digital signals,and so forth, a central processing unit (hereinafter called "the CPU")5b, memory means 5c storing various operational programs which areexecuted in the CPU 5b and for storing results of calculationstherefrom, etc., and an output circuit 5d which outputs driving signalsto the fuel injection valves 6, the driving circuit 21, and theelectromagnetic valve 23.

The CPU 5b operates in response to the above-mentioned signals from thesensors to determine operating conditions in which the engine 1 isoperating such as an air-fuel ration feedback control region forcontrolling the air-fuel ratio in response to oxygen concentration inexhaust gases and open-loop control regions, and calculates, based uponthe determined operating conditions, the valve opening period or fuelinjection period T_(OUT) over which the fuel injection valves 6 are tobe opened, by the use of the following equation in synchronism withinputting of TDC signal pulses to the ECU 5.

    T.sub.OUT =Ti×K.sub.WOT ×K.sub.1 +K.sub.2      (1)

where Ti represents a basic fuel amount, more specifically a basic fuelinjection period of the fuel injection valves 6, which is determinedbased upon the engine rotational speed Ne and the intake pipe absolutepressure P_(BA). As the Ti map for determining the Ti value, a Ti_(L)map for low speed valve timing and a Ti_(H) map for high speed valvetiming are stored in the memory means 5C.

K_(WOT) represents a high load enriching coefficient for increasing theamount of fuel in a predetermined high load engine operating region.

K₁ and K₂ represent other correction coefficients and correctionvariables, respectively, which are calculated based on various engineparameter signals to such values as to optimize operatingcharacteristics of the engine such as fuel consumption andaccelerability, depending on operating conditions of the engine.

The CPU 5b decides the ignition timing θig based on the enginerotational speed Ne and the intake pipe absolute pressure P_(BA). As theθig map for determining the ignition timing, similarly to the Ti maps, aθgi_(L) map for the low speed valve timing and a θig_(H) map for thehigh speed valve timing are stored in the memory means 5C.

Further, the CPU 5b controls the opening and closing of theelectromagnetic valve 23 in a manner described hereinafter withreference to FIG. 9.

The CPU 5b supplies the output circuit 5d with driving signals fordriving the fuel injection valves 6, the driving circuit 21, and theelectromagnetic valve 23, based upon the results of the abovecalculations and decisions.

FIG. 2 shows a longitudinal cross-section of essential parts of theengine 1. Four cylinders 32, only one of which is shown, are arranged ina line within a cylinder block 31. Combustion chambers 35 are definedbetween a cylinder head 33 mounted on an upper end of the cylinder block31 and pistons 34 slidably fitted within respective cylinders 32. Thecylinder head 33 has a pair of inlet ports 36 and a pair of exhaustports 37 formed in a portion thereof serving as a ceiling of eachcombustion chamber. Each inlet port 36 is connected to an inlet passage38 which opens in one side wall of the cylinder head 33, while eachexhaust port 37 is connected to an exhaust passage 39 which opens inanother side wall of the cylinder head 33.

An inlet valve 40i is arranged in each inlet part 36 to open and closesame, while an exhaust valve 40e is arranged in each exhaust port 37 toopen and close same. The inlet valves 40i and exhaust valves 40e areguided by respective guide sleeves 41i' and 41e' which are fitted inrespective guide holes 41i and 41e formed in the cylinder head 33. Valvesprings 43i, 43e are interposed between respective valve seats formed atends of the guide holes 41i, 41e and respective collars 42i, 42e securedon upper ends of each inlet valve 40i and each exhaust valve 40eprojected from the respective guide holes 41i, 41e. The valve springs43i, 43e urge the respective inlet and exhaust valves 40i, 40e in theupward or valve-closing direction.

The cylinder head 33 and a head cover 44 mounted on an upper end thereofdefine therebetween a working chamber 45 which accommodates an inletvalve-operating device 47i for opening and closing the inlet valve 40iin each cylinder 32, and an exhaust valve-operating device 47e foropening and closing the exhaust valve 40e in same. The valve-operatingdevices 47i, 47e are basically of the same construction. Therefore, onlythe component parts of the inlet valve-operating device 47i will bedescribed below with reference numerals having a letter i affixedthereto, while those of the exhaust valve-operating device 47e aremerely shown in the drawings with corresponding reference numeralshaving a letter e affixed thereto.

Referring also to FIG. 3, the inlet valve-operating device 47i comprisesa camshaft 48i rotatatively driven by a crankshaft, not shown, at aspeed ratio of 1/2, a high speed cam 51i and low speed cams 49i, 50isecured on the camshaft 48i, provided for each cylinder 32 (the lowspeed cam 50i has substantially the same configuration as the low speedcam 49i, both being arranged on opposite sides of the high speed cam51i), a rocker shaft 52i extending parallel with the camshaft 48i, firstand second driving rocker arms 53i and 54i, and a free rocker arm 55ipivotally mounted on the rocker shaft 52i, the three arms being providedfor each cylinder 32, a connection-changeover mechanism 56i arranged inthe corresponding rocker arms 53i, 54i, 55i for each cylinder.

As shown in FIG. 3, the connection-changeover mechanism 56i comprises afirst changeover pin 81 capable of connecting the first driving rockerarm 53i with the free rocker arm 55i, a second changeover pin 82 capableof connecting the second driving rocker arm 54i with the free rocker arm55i, a restriction pin 83 for restricting the movement of the first andsecond changeover pins 81, 82, and a spring 84 urging the pins 81, 82,83 in the rocker arms-disconnecting direction.

The first driving rocker arm 53 i if formed therein with a first guidebore 85 extending parallel with the rocker shaft 52i with one endthereof closed and the other end opening in a side face thereof facingthe free rocker arm 55i. The first changeover pin 81 is slidably fittedin the first guide bore 85, defining an oil hydraulic chamber 86 betweenone end thereof and the closed end of the first guide bore 85. Further,a passage 87 extends from the oil hydraulic chamber 86 and opens into anoil feeding passage 88i formed in the rocker shaft 52i so that thepassage 88i permanently communicates via the passage 87 with the oilhydraulic chamber 86 irrespective of rocking motion of the first drivingrocker arem 53i.

The free rocker 55i is formed therein with a guide through hole 89 at alocation corresponding to the first guide bore 85, which extends throughthe free rocker arm 55i and parallel with the rocker shaft 52i. Thesecond changeover pin 82 is slidably fitted in the guide through hole89, with one end thereof abutting on an opposed end face of the firstchangeover pin 81.

The second driving rocker arm 54i is formed therein with a second guidebore 90 at a location corresponding to the guide through hole 89, whichextends parallel with the rocker shaft 52i with one end thereof openingtoward the free rocker arm 55i. The restriction pin 83 in the form of adisc is slidably fitted in the second guide bore 90, in a fashionabutting on the other end of the second changeover pin 82. Further, thesecond guide bore 90 has a guide sleeve 91 fitted therein, in which isslidably fitted an axial rod 92 which coaxially and integrally projectsfrom the restriction pin 82. The spring 84 is interposed between theguide sleeve 91 and the restriction pin 83 and urges the pins 81, 82, 83toward the oil hydraulic chamber 86.

In the connection-changeover mechanism 56i constructed as above, whenthe pressure in the oil hydraulic chamber 86 is increased, the firstchangeover pin 81 is forced to move into the guide through hole 89 andat the same time the second changeover pin 82 is urgedly moved into thesecond guide bore 90 to connect the rocker arms 53i, 55i, 54i together.When the pressure in the oil hydraulic chamber 86 is decreased, thefirst changeover pin 81 is moved back by the urging force of the spring84 into a position in which the end face thereof abutting on the secondchangeover pin 82 corresponds in location to the space between the firstdriving rocker arm 53i and the free rocker arm 55i, and at the same timethe second changeover pin 82 is moved back into a position in which theend face thereof abutting on the restriction pin 83 corresponds inlocation to the space between the free rocker arm 55i and the seconddriving rocker arm 54i, whereby the rocker arms 53i, 55i, 54i becomedisconnected from each other.

Next, the oil-feeding system for supplying oil to the valve-operatingdevices 47i, 47e will be described with reference to FIGS. 4A-4C. Oilgalleries 98, 98' are connected to an oil pump, not shown, for pumpingoil from an oil pan, not shown. From the oil galleries 98, 98', oilpressure is supplied to the connection-changeover mechanisms 56i, 56e,while lubricating oil is supplied to the lubricating parts of thevalve-operating devices 47i, 47e.

Connected to the oil gallery 98 is a selector valve 99 for changing theoil pressure supplied between high and low levels. The oil feedingpassages 88i, 88e in the respective rocker shafts 52i, 52e are connectedvia the selector valve 99 with the oil gallery 98.

Passage-forming members 102i, 102e respectively extend parallel with thecamshafts 48i, 48e and are secured to upper surfaces of cam holders 59by means of a plurality of bolts. The passage-forming members 102i, 102eare formed therein with respective low speed lubricating oil passages104i, 104e, and high speed lubricating oil passages 105i, 105e, all ofwhich have opposite closed ends and form pairs of parallel passages. Thelow speed lubricating oil passages 104i, 104e are connected to the oilgallery 98', and the high speed lubricating oil passages 105i, 105e areconnected to the oil feeding passages 88i, 88e. Further, the low speedlubricating oil passages 104i, 104e are connected to the cam holder 59.

The selector valve 99 comprises an oil inlet port 119 connected to theoil gallery 98, an oil outlet port 120 connected to the oil feedingpassages 88i, 88e, and a spool valve 122 slidably fitted within ahousing 121 mounted on one end face of the cylinder head 33.

The housing 121 is formed therein with a cylinder hole 124 having anupper end thereof closed with a cap 123, and within which is fitted thespool valve 122 to define an operating oil hydraulic chamber 125 betweenan upper end thereof and the cap 123. Further, a spring 127 isaccommodated within a spring chamber 126 defined between the spool valve122 and a lower part of the housing 121 and urges the spool valve 122 inthe upward or valve-closing direction. The spool valve 122 has anannular recess 128 formed therearound for communicating between the oilinlet port 119 and the oil outlet port 120. When the spool valve is inan upper position as shown in FIG. 4, it cuts off the communicationbetween the oil inlet port 119 and the oil outlet port 120.

An oil filter 129 is held between the oil inlet port 119 and a highspeed oil pressure feeding passage 116. Further, the housing 121 has arestriction passage 131 formed therein, which provides communicationbetween the oil inlet port 119 and the oil outlet port 120. Therefore,even when the spool valve 122 is in a closed position, the oil inletport 119 and the oil outlet port 120 are communicated with each otherthrough the restriction passage 131 whereby oil pressure decreasedthrough the restriction passage 131 is supplied via the oil outlet port120 to the oil feeding passages 88i, 88e.

The housnig 121 also has a bypass port 132 formed therein, which isdisposed to communicate via the annular recess 128 with the oil outletport 120 only when the spool valve 22 is in the closed position. Thebypass port 132 communicates with the interior of the cylinder heat 33at an upper location thereof.

Connected to the housing 121 is a conduit line 135 which alwayscommunicates with the oil inlet port 119. The conduit line 135 isconnected via the electromagnetic valve 23 with a conduit line 137 whichis in turn connected with a communcation hole 138 formed in the cap 123.Therefore, when the electromagnetic valve 23 is opened, oil pressure issupplied to the operating oil hydraulic chamber 125 to thereby move thespool valve 122 in the valve-opening direction.

Further, the housing 121 has the oil pressure sensor 18 mounted thereonfor detecting the oil pressure in the outlet port 120, i.e. the oilpressure in oil feeding passages 88i, 88e, to determine whether theselector valve 99 is normally functioning or not.

The operation of the valve-operating devices 47i, 47e having theabove-described construction will be described below. Since thevalve-operating devices 47i, 47e operate similarly to each other, thefollowing description refers only to the operation of the inletvalve-operating device 47.

When the ECU 5 sends out a valve-opening instruction signal to theelectromagnetic valve 23, the electromagnetic valve 23 is opened tothereby cause the selector valve 99 to open, so that the oil pressure inthe oil feeding passage 88i is increased. This causes theconnection-changeover mechanism 56i to operate to connect the rockerarms 53i, 54i, 55i together, whereby the high speed cam 51i operates therocker arms 53i, 54i, 55i in unison to cause each pair of inlet valves40i to open and close at high speed valve timing in which thevalve-opening period and the valve lift amount are relatively greater.

On the other hand, when the ECU 5 supplies a valve-closing instructionsignal to the electromagnetic valve 23, the electromagnetic valve 23 andin turn the selector valve 99 are closed to thereby decrease the oilpressure in the oil feeding passage 88i. This causes theconnection-changeover mechanism 56i to operate to disconnect the rockerarms 53i, 54i, 55i from each other, whereby the low speed cams 49i, 50ioperate the corresponding rocker arms 53i, 54i to cause the pair ofinlet valves 40i to open and close at low speed valve timing in whichthe valve-opening period and the valve lift amount are relativelysmaller.

Next, the valve timing-changeover control according to the inventionwill be described below.

In FIG. 5, the Ti value of the Ti_(L) map for low speed valve timing andthat of the Ti_(H) map for high speed valve timing are respectivelyindicated by solid lines and dotted lines. As is clear from the figure,in the case of the low speed valve timing being selected, the rate ofincrease in the intake air amount becomes smaller with increase in theengine rotational speed Ne, while in the case of the high speed valvetiming being selected, the charging efficiency becomes higher withincrease in the engine rotational speed Ne whereby the intake air amountbecomes greater than in the case of the low speed valve timing beingselected. Therefore, there is a point of engine rotational speed Newhere the Ti value for low speed valve timing and the Ti value for highspeed valve timing are identical to each other. In this point of Ne, inboth cases of the high and low speed valve timings being selected, theintake air amounts is identical, and at the same time the air-fuel ratiois also identical, so that the engine output becomes substantiallyidentical.

The charging efficiency finely varies with the engine rotational speedNe, and particularly in the vicinity of the maximum throttle valveopening (θth), the variation becomes markedly great. FIG. 6 shows on anenlarged scale part of FIG. 5 for explaining this variation. At aplurality of points, the Ti value for low speed valve timing and thatfor high speed valve timing become identical to each other. As describedhereinafter, when the valve timing is changed at a point where the Tivalue for low speed valve timing and that for high speed valve timingare identical to each other, hunting in changeover the valve timing,i.e. frequent changeover of valve timing, is liable to occur in theregion of wide throttle valve opening (WOT), which adversely affects thedurablity of the connection-changeover mechanisms 56i, 56e.

In this connection, when the engine is in a high load operating region(maximum θth (WOT) region), the air-fuel ratio is enriched by the highload enriching coefficient K_(WOT) to increase the engine output. Insuch a high load operating region, the engine output can be moreeffectively increased if the valve timing is changed to the high speedvalve timing. Therefore, when the engine is in the high load operatingregion (maximum θth (WOT) region), from a T_(VT) table in which a highload determination value T_(VT) experimetally obtained based on the fuelinjection amount T_(OUT) is set in relation to the engine rotationalspeed Ne as shown in FIG. 7, a T_(VT) value is obtained in accordancewith the engine rotational speed Ne, and when the fuel injection amountT_(OUT) is equal to or higher than the T_(VT) value, the valve timing ischanged to the high speed valve timing. In this case, if it is arrangedthat the region defined by T_(OUT) ≧T_(VT) includes the aforesaid pointsin the wide throttle valve opening region where the Ti value for lowspeed valve timing and that for high speed valve timing are identical toeach other, the hunting in the valve timing can be prevented. Inaddition, the T_(VT) table used for vehicles with automatictransmissions is different from that used for vehicles with manualtransmissions.

Further, generally, in order to prevent overspeed of the engine, fuelcut is carried out when the engine rotational speed Ne exceeds apredetermined value (so-called revolution limitter value) N_(HFC). Loadacting on a timing belt connecting between the crankshaft and thecamshaft increases with decrease in the opening period of the valvebecause the acceleration of opening movement of the valve increases withdecrease in the valve opening period. Further, as the accelerationincreases, a critical value of the engine rotation speed above whichthere can occur jumping of the valve decreases. Therefore, the maximumallowable engine rotational speed should be different between when thevalve timing is set to the low speed valve timing in which the valveopening period is shorter and when the valve timing is set to the highspeed valve timing in which the valve opening period is longer.Accordingly, in this embodiment, the revolution limitter value is set ata relatively low value N_(HFC1) (e.g. 7500 rpm) for the low speed valvetiming, and at a relatively high value N_(HFC2) (e.g. 8100 rpm) for thehigh speed valve timing.

Next, reference is made to FIG. 8 showing the valve timing regions. Inthe figure, the solid line indicates a boundary line between the lowspeed valve timing region and the high speed valve timing region, whichis selected when the valve timing is changed from the low speed valvetiming to the high speed valve timing, and the broken line indicates onewhich is selected when the valve timing is changed from the high speedvalve timing to the low speed valve timing.

The changeover of the valve timing is carried out in a region between avalue Ne₁ of the engine rotational speed below which the engine outputobtained by the low speed valve timing always exceeds the engine outputobtained by the high speed valve timing and a value Ne₂ of the enginerotational speed above which the engine output obtained by the highspeed valve timing always exceeds the engine output obtained by the lowspeed valve timing. In this embodiment, hysteresis is imparted to theengine rotational speed values Ne₁ and Ne₂ between changeover of thevalve timing from the low speed valve timing to the high speed valvetiming and vice versa such that Ne₁ is set to e.g. 4800 rpm/4600 rpm andNe₂ set to e.g. 5900 rpm/5700 rpm.

In FIG. 8, X indicates a region in which the engine is in a high loadoperating region (WOT region) and the changeover of valve timing iscarried out by comparison between T_(OUT) and T_(VT), and Y indicates aregion in which the changeover of the valve timing is carried out bycomparison between a T_(IL) value for the low speed valve timing and aT_(IH) value for the high speed valve timing. Incidentally, since thechangeover characteristic in the region X is also under the influence ofparameters other than the engine rotational speed Ne and the intake pipeabsolute pressure P_(BA) used for calculation of T_(OUT), the changeovercharacteristic cannot be accurately plotted in FIG. 8 in which theengine rotational speed Ne is indicated by the abscissa and the intakepipe absolute pressure P_(BA) is indicated by the ordinate. Therefore,the changeover characteristic in the region X of FIG. 8 should be takenas one for mere understanding of the concept of the invention.

Next, reference is made to FIGS. 9A-9C to explain a program forcontrolling the changeover of the valve timing executed by the ECU 5,i.e. a program for output control of signals supplied to theelectromagnetic valve 23. This program is executed upon generation ofeach pulse of the TDC signal and in synchronism therewith.

At a step 901, it is determined whether or not a failsafe operationshould be carried out, e.g. by determining whether or not any engineoperating parameter sensor is normally functioning or whether or notabnormality has occurred in the control system other than such sensor.

Specifically, it is determined that the engine is in an operatingcondition in which a failsafe operation should be carried out, if, forexample, there is detected an abnormality in any of the outputs from theintake pipe absolute pressure (P_(BA)) sensor 8, thecylinder-discriminating (CYL) sensor 12, the engine rotational speed(TDC) sensor 11, the engine coolant temperature sensor 10, and thevehicle speed sensor 16, an abnormality in outputting of a controlsignal for ignition timing or in outputting of driving signals for thefuel injection valves, an abnormality in the amount of electric currentsupplied to the electromagnetic valve 23 for the valve timing control,or an abnormality that a normal change has not been detected in oilpressure at the oil outlet port 120 responsive to opening and closing ofthe electromagnetic valve 23 for the valve timing control by an oilpressure switch of the oil pressure sensor 18, over a predetermined timeperiod. In addition, when one of the CYL sensor and the TDC sensor isabnormal, the other is used in place thereof.

At a step 902, it is determined whether or not the engine is beingstarted, from the engine rotational speed Ne, etc., and at a step 903,it is determined whether or not a delay timer has counted up apredetermined time period (e.g. 5 seconds) t_(ST). If the answer to thequestion of the step 902 is Yes, the program proceeds to a step 904,where the timer is set to the predetermined time period t_(ST) duringfailsafe operation or engine starting, for starting to count same afterthe engine starting has been completed. At a step 905, it is determinedwhether or not the engine coolant temperature T_(W) is lower than apredetermined value T_(W1) (e.g. 60°C.), i.e. whether or not the engineis in warming-up operation. At a step 906, it is determined whether ornot the vehicle speed V is lower than a very low predetermined value V₁(with hysteresis, e.g. 8 km/5 km). At a step 907, it is determinedwhether or not the vehicle on which the engine is installed is providedwith a manual transmission (MT). At a step 908, it is determined, whenthe vehicle is an automatic transmission type (AT), whether or not theshift lever is positioned in the parking range (P) or the neutral range(N). At a step 909, it is determined whether or not the enginerotational speed Ne is not lower than the predetermined lower limitvalue Ne₁ (e.g. 4800 rpm/4600 rpm). If as a result of the abovedeterminations, the failsafe operation is being carried out (the answerto the question of the step 901 is Yes), or if the engine is beingstarted (the answer to the question of the step 902 is Yes), or if thepredetermined time period t.sub. ST has not elapsed after the enginerhas completed starting (the answer to the question of the step 903 isNo), or if the engine is still in warming up operation (the answer tothe question of the step 905 is Yes), or if the vehicle is standing ormoving slowly (the answer to the question of the step 906 is Yes), or ifthe shift lever is in the P or N range (the answer to the question ofthe step 908 is Yes), or if Ne<Ne₁ (the answer to the question of thestep 909 is No), the electromagnetic valve 23 is closed to maintain thelow speed valve timing.

If it is determined at the step 909 that Ne≧Ne₁ is satisfied, at a step910, from the Ti_(L) map and the Ti_(H) map, there are obtained a Tivalue (hereinafter referred to as "Ti_(L) ") of the Ti_(L) map and a Tivalue (hereinafter referred to as "Ti_(H) ") of the Ti_(H) map eachcorresponding to the engine rotational speed Ne and the intake airabsolute pressure P_(BA). Then, at a step 911, from the T_(VT) table setdepending on whether the vehicle is AT or MT is obtained a high loaddetermination value T_(VT) corresponding to the engine rotational speedNe. At a step 912, the T_(VT) is compared with the T_(OUT) in theimmediately preceding loop to determine whether T_(OUT) ≧T_(VT) issatisfied, i.e. whether the engine is in the high load operatingcondition in which the air-fuel ratio should be enriched. If the step912 is No, i.e. if T_(OUT) <T_(VT) is satisfied, the program proceeds toa step 913, where it is determined whether or not the engine rotationalspeed Ne is not lower than the predetermined upper limit value Ne₂. Ifthe answer to the question of the step 913 is No, i.e. if Ne<Ne₂ issatisfied, the program proceeds to a step 914, where the Ti_(L) and theTi_(H) obtained at the step 910 are compared with each other. If Ti_(L)>Ti_(H) is satisfied, it is determined at a step 916 whether or not atimer value t_(VTOFF) of a delay timer set at a step 915, referred tohereinafter, has been counted up. If the answer to the question of thestep 916 is Yes, an instruction signal for closing the electromagneticvalve 23, i.e. an instruction for changing the valve timing to the lowspeed valve timing, is generated at a step 917. On the other hand, ifany of T_(OUT) ≧T_(VT), Ne≧Ne₂, and Ti_(L) ≦Ti_(H) is satisfied, thedelay timer for closing the electromagnetic valve is set to thepredetermined value t_(VTOFF) (e.g. 3 seconds) and started at the step915. Then at a step 918, an instruction signal for opening theelectromagnetic valve 23, i.e. an instruction for changing the valvetiming to the high speed valve timing is generated.

If the valve-closing signal is generated at the step 917, it isdetermined at a step 919 whether or not the oil pressure switch withinthe oil pressure sensor 18 has been turned on, i.e. if the oil pressurein the oil feeding passages 88i, 88e has become low. If the answer tothe question of the step 919 is Yes, i.e. if the oil pressure switch hasbeen turned on, it is determined at a step 921 whether or not achangeover delay timer has counted up a predetermined time periodt_(LVT) for the low speed valve timing. If the answer to the question ofthe step 921 is Yes, i.e. if t_(LVT=) 0, another changeover delay timerfor the high speed valve timing is set to a predetermined time periodt_(HVT) (e.g. 0.1 second) and started at a step 923. Then at a step 925,the Ti_(L) map and an ignition timing map (θig_(L) map) for the lowspeed valve timing are selected as the Ti map and the ignition timingmap to be used in a routine for fuel injection control. At the followingstep 927, the revolution limitter value N_(HFC) is set to apredetermined value N_(HFC1) for the low speed valve timing.

On the other hand, if the valve-opening signal is generated at the step918, it is determined at a step 920 whether or not the oil pressureswitch within the oil pressure sensor 18 has been turned off, i.e. ifthe oil pressure in the oil feeding passages 88i, 88e has become high.If the answer to the question of the step 920 is Yes, i.e. if the oilpressure switch has been turned off, it is determined at a step 922whether or not the changeover delay timer for the high speed valvetiming has counted up the value t_(HVT). If the answer to the questionof the step 922 is Yes, i.e. if t_(HVT=) 0, the the changeover delaytimer for the low speed valve timing is set to a predetermined timeperiod t_(LVT) (e.g. 0.2 seconds) at a step 924, and then at a step 926,the Ti_(H) map and an ignition timing map (θig_(H) map) for the highspeed valve timing are selected as the Ti map and the ignition timingmap to be used in the routine for fuel injection control. At thefollowing step 928, the revolution limitter value N_(HFC) is set to apredetermined value N_(HFC2) for the high speed valve timing, which ishigher than N_(HFC1).

The predetermined delay time periods t_(HVT) and t_(LVT) are set at suchvalues as correspond to the respective time lags, i.e. periods of timeto be elapsed from opening and closing of the electromagnetic valve 23,through switching of the selector valve 99, and changing of the oilpressure in the oil feeding passages 88i, 88e, until completion ofchangeover operations by the connection-changeover mechanisms 56i, 56eof all the cylinders. When the electromagnetic switch 23 is closed, theprogram proceeds in the order of 919-922-924-926-928 until the oilpressure switch within the oil pressure sensor 18 is turned on. Afterthe oil pressure switch has been turned on, the program proceeds in theorder of 919-921-926-928 until the connection-changeover mechanisms 56i,56e of all the cylinders have been changed over to the low speed valvetiming position. Further, if the selector valve 99 is not closed due tofailure of the electromagnetic valve 23 or the selector valve 99 etc. sothat the oil pressure switch within the oil pressure sensor 18 remainsopen or off, the program also proceeds in the above-mentioned order of919-922-924-926-928. Thus, until the connection-changeover mechanisms56i, 56e of all the cylinders have been changed to the low speed valvetiming position, the fuel injection is controlled in a manner suitablefor the high speed valve timing. Also, when the electromagnetic switch23 is opened, the fuel injection is controlled in a manner suitable forthe low speed valve timing until the connection-changeover mechanisms56i, 56e of all the cylinders have been changed to the high speed valvetiming position.

In the meanwhile, if failsafe operation is being carried out (the answerto the question of the step 901 is Yes), or if the engine is beingstarted (the answer to the question of the step 902 is Yes), or if thetime period t_(ST) has not elapsed after completion of the enginestarting (the answer to the question of the step 903 is No), or if theengine has not yet been warmed up (the answer to the question of thestep 905 is Yes), or if the vehicle is standing or moving slowly (theanswer to the question of the step 906 is Yes), the program proceeds tothe step 929, where the instruction signal for closing theelectromagnetic valve 23 is generated, followed by the programproceeding in the order of 923-925-927. If it is determined at the step908 that the shift lever position is in the N or P range, the programproceeds to a step 930, where it is determined whether or not the Ti_(H)map has been selected in the immediately preceding loop. Also, if it isdetermined at the step 909 that Ne<Ne₁ is satisfied, the programproceeds to the step 930. If the answer to the question of the step 930is Yes, i.e. if the Ti_(H) map has been selected in the immediatelypreceding loop, the time period t_(VTOFF) of the delay timer over whichthe electromagnetic valve is to be opened is set to 0 at a step 931, andthen the program proceeds to a step 917. If the answer to the questionof the step 930 is No, i.e. if the Ti_(H) map has not been used in theimmediately preceding loop, in other words, if the connection-changeovermechanisms 56i, 56e of all the cylinders have not been changed over tothe high speed valve timing position, the program proceeds, as describedabove, in the order of 929-923-925-927, whereby the fuel injection iscontrolled in a manner suitable for the low speed valve timingirrespective of the state of the oil pressure switch within the oilpressure sensor 18. This is a countermeasure for the case in which theoil pressure switch within the oil pressure sensor 18 continues to beoff due to disconnection in the wiring etc.

The aforesaid revolution limitter Ne value N_(HFC1) is set at a valuehigher than Ne₂, and normally the valve timing is switched to the highspeed valve timing and accordingly the revolution limitter N_(HFC) isset to the higher vlaue N_(HFC2) before the engine rotational speed Nerises to N_(HFC1), so that fuel cut is not carried out even at N_(HFC1).On the other hand, when the engine is in an operating condition in whichthe program proceeds from any of the steps 902-906, and 908 to the step929, the fuel cut can be carried out at N_(HFC1), since the low speedvalve timing is maintained even after the engine rotational speed Neexceeds Ne₂ due to racing of the engine etc. Further, even after thevalve timing is switched from the low speed valve timing to the highspeed valve timing, fuel cut is carried out at N_(HFC1) before t_(HVT)becomes 0, i.e. before the connection-changeover mechanisms 56i, 56e areactually changed over to the high speed valve timing position.

FIG. 10 shows the subroutine used at the step 910 for obtaining Ti_(L)and Ti_(H) from the respective Ti_(L) and Ti_(H) maps. It is determinedwhether or not the instruction signal for opening the electromagneticswitch 23 has been generated in the immediately preceding loop. If theinstruction signal has not been generated, the Ti_(L) to be used at thestep 914 is set to a value Ti_(L) obtained from the Ti_(L) map, whereasif the instruction signal has been generated, the Ti_(L) to be used atthe step 914 is set to a value obtained by subtracting a predeterminedhysteresis amount of ΔTi from a valve Ti_(L) obtained from the Ti_(L)map. Thus, hysteresis is imparted to the changeover characteristic inthe region Y in FIG. 8.

FIG. 11 shows the subroutine used at the step 911 for obtaining the highload determination value T_(VT) from the T_(VT) table. It is determinedwhether or not the instruction signal for opening the electromagneticvalve 23 has been generated in the immediately preceding loop. If thesignal has not been generated, the T_(VT) to be used at the step 912 isset to a value T_(VT) obtained from the T_(VT) table, whereas if thesignal has been generated, the T_(VT) to be used at the step 912 is setto a value obtained by subtracting a predetermined hysteresis amountΔT_(VT) from a value T_(VT) obtained from the T_(VT) table. Thus,hysteresis is imparted to the changeover characteristic in the region Xin FIG. 8.

Referring again to FIG. 9, if the answer to the question of the step 901is Yes, i.e. if the failsafe operation is being carried out, theinstruction signal for closing the electromagnetic valve 23 is generatedat the step 932, and then at a step 933 it is determined whether or notthe engine rotational speed Ne is higher than a predetermined valueNe_(FS) for fail-safe (e.g. 3,000 rpm). If the answer at the step 933 isYes, i.e. if Ne>Ne_(FS), the Ti_(H) map and the θig_(H) map for the highspeed valve timing are selected at a step 934, followed by the programproceeding to the step 927, whereas if Ne≦Ne_(FS), the Ti_(L) map andthe θig_(L) map for the low speed valve timing are selected at a step935, followed by the program proceeding to the step 927.

As described above, at the steps 933-935 one of the Ti_(H) Ti_(L) mapsis selected depending on the engine rotational speed Ne during thefail-safe operation. Therefore, even when the inlet and exhaust valves40i, 40e are actually operated at the high speed valve timing due tofailure of any of the selector valve 99, the connection-changeovermechanisms 56i, 56e, etc. in spite of the fact that the instructionsignal for closing the electromagnetic valve 23 is generated during thefailsafe operation, it is possible to prevent overleaning of theair-fuel ratio and hence an excessive rise in the buring temperature ofthe mixture or the catalyst temperature of exhaust gas purifying means,and accordingly also prevent melting of ignition plugs due topreignition of the mixture, knocking at a high engine rotational speed,and shortened life of the catalyst.

The manner of ignition timing control which is carried out by theknocking control system of the invention will now be explained withreference to FIG. 12.

At a step 1201, the detected and stored engine rotational speed Ne andintake pipe absolute pressure P_(BA) are read out, and at a step 1202 abasic ignition timing advancing amount θig_(-BASE) is retrieved from theθig_(L) or θig_(H) map selected at the step 925 or 926 in FIG. 9, inresponse to the engine rotational speed Ne and the intake pipe absolutepressure P_(BA) read out at the step 1201. Then, the engine coolanttemperature T_(W) and the intake air temperature T_(A) are read out at astep 1203, and at a step 1204 a correction variable θig_(-COR) forcorrecting the basic ignition timing advancing amount θig_(-BASE) iscalculated based on the engine coolant temperature T_(W) and the intakeair temperature T_(A) read out at the step 1203. At a step 1205, acorrection variable θig_(KN) for preventing knocking is read out, whichis determined depending on the valve timing and the result of adetermination as to occurrence of knocking, hereinafter referred to, andthen stored. A final ignition timing advancing amount θig (namely acrank angle before the top-dead-center of each cylinder at the start ofcompression stroke) is calculated at a step 1206 by summing up the valueθig_(-BASE), θig_(-COR) , and θig_(KN) obtained at the steps 1202, 1204,and 1205, and at a step 1207 an ignition signal based on the calculatedvalue θig is supplied to the driving circuit 21 to effect ignition bythe ignition plug 22.

The determination as to whether knocking has occurred, which is used fordetermining the correction variable θig_(KN) read out at the step 1025,is carried out by the use of the knocking sensor 24 and means 1301-1305shown in (b) of FIG. 13. Specifically, an output from the knockingsensor 24 which is indicative of engine vibration is applied to gatemeans 1301, where the sensor output is subjected to detection by aknocking gate and a noise gate of the gate means 1301, as shown in (a)of FIG. 13, at both the low speed valve timing (hereinafter referred toas "Lo V/T") and the high speed valve timing (hereinafter referred to as"Hi V/T"). Vibration detected by the noise gate is levelled through alow-pass filter having a predetermined time constant, of noiselevel-calculation means 1302, to obtain mean noise levels NL_(L), NL_(H)at Lo V/T and Hi V/T, respectively. The mean noise levels NL_(L), NL_(H)are multiplied by multiplier terms G_(L), G_(H) and increased by addenedterms OS_(L), OS_(H) by the following equations (2) and (3) at knockingdiscrimination calculating means 1303, to obtain respective knockingdiscrimination levels aLo, aHi at Lo V/T and Hi V/T.

    aLo=NL.sub.L ×G.sub.L +OS.sub.L                      (2)

    aHi=NL.sub.H ×G.sub.H +OS.sub.H                      (3)

where the multiplier terms G_(L), G_(H) and addend terms OS_(L), OS_(H)are set at Lo V/T and Hi V/T, respectively, and obtained from graphsshown in FIG. 17, respectively. The multiplier terms and addend termsare both set to larger values as the engine rotational speed Ne or theengine load increases. Further, the multiplier term G_(H) and addendterm OS_(H) at Hi V/T are set to larger values than then the multiplierterm G_(L) and addend term OS_(L) at Lo V/T. Also, the increasing ratesof the multiplier term G_(L) and addend term OS_(L) at Lo V/T becomereduced when the engine rotational speed Ne or engine load exceeds achageover point CH, e.g. 4800 rpm. By thus setting the multiplier termsG_(L), G_(H) and addend terms OS_(L), OS_(H), the knockingdiscrimination levels aLo, aHi at Lo V/T and Hi V/T are substantiallyequal to each other and at the same time set in the vicinity of a lowerlimit value of a knocking level region, in spite of the fact that thenoise level NL_(L) at Lo V/T is different from the noise level NL_(H) atHi V/T, as shown in FIG. 18.

In practice, the knocking discrimination level calculating means iscomposed of amplifiers adapted such that the multiplier terms G_(L),G_(H), and the addend terms OS_(L), OS_(H) are determined by the gainsand offset of the amplifiers, respectively, in dependence on the enginerotational speed Ne or engine load, as well as on the selected valvetiming.

Then, the vibration levels detected by the knocking gate at both Lo V/Tand Hi V/T are compared with the respective knocking discriminationlevels aLo and aHi by means of knocking discriminating means 1304. Ifthe former is higher than the later, the knocking discriminating means1304 judges that knocking has occurred, and then supplies an electricsignal indicative of occurrence of knocking to knocking control means1305.

The manner or knocking control will now be described with reference toFIG. 14.

At a step 1401, it is determined whether or not there exists a failurein the value timing control means, that is, it is determined similarlyto the step 901 in FIG. 9 whether or not the failsafe operation shouldbe carried out. If the answer at the step 1401 is No, that is, if thereis no failure in the valve timing control means, the program proceeds toa step 1402, where it is determined whether or not the actual valvetiming is Lo V/T. If at the step 925 in FIG. 9 the Lo V/T map has beenselected, the valve timing is judged to be Lo V/T, whereas if at thestep 926 the Hi V/T map has been selected, the valve timing is judged tobe Hi V/T.

If the answer at the step 1402 is Yes, that is, if Lo V/T is beingapplied, various control data for Lo V/T are selected at a step 1403,whereas if the answer is No, that is, if Hi V/T is being applied,various control data for Hi V/T are selected at a step 1404.

The various control data include: the multiplier terms G_(L), G_(H), andthe addend terms OS_(L), OS_(H) to be used at a step 1405, hereinafterreferred to, for calculation of knocking discrimination levels;discrimination parameters to be used in the knocking discriminatingmeans 1304 such as crank angles at which the knocking gate and noisegate are operated, the time constant of the low-pass filter forlevelling noise levels, and a reference discrimination level used fordetecting abnormality in the knocking sensor 24; and control parametersto be used in the knocking control means 1305 such as a predeterminedretarding limit value θig_(KNRD) in FIG. 15, a predetermined advancinglimit value θig_(KNAV) in FIG. 16, the frequency of occurrence ofknocking used for determining the octane number of fuel (the octanenumber of fuel used is determined based on the knocking frequency, andthe retarding amount of the ignition timing is set depending on theoctane number), a predetermined retarding correction amount Δθ_(RD) inFIG. 15, and a predetermined advancing correction amount Δθ_(AV) in FIG.16.

At the step 1405, the knocking discrimination levels aLo and aHi arecalculated by means of the knocking discrimination level-calculatingmeans 1303 (FIG. 13) on the basis of the multiplier terms G_(L), G_(H)and addend terms OS_(L), OS_(H) for Lo V/T and Hi V/T, respectively,from the various data selected at the steps 1403, 1404. At a step 1406,it is determined whether or not the output of the knocking sensor 24detected by the knocking gate is higher than corresponding one of thelevels aLo and aHi calculated at the step 1405. If the answer is Yes,which indicates that there exists knocking, the program proceeds to astep 1407 to calculate an ignition timing retarding value or ignitiontiming correction variable θig_(KN) in accordance with a subroutine ofFIG. 15, whereas if the answer is No, which indicates that there is noknocking, the program proceeds to a step 1408 to calculate an ignitiontiming advancing value (θig_(KN)) in accordance with a subroutine ofFIG. 16.

Referring again to the step 1401, if the answer is Yes, that is, ifthere exists a failure in the valve timing control means and hence thefailsafe operation should be carried out, the program proceeds to a step1409 where the failsafe operation for knocking is carried out.

The failsafe operation is carried out in accordance with a subroutineshown in FIG. 20. That is, similarly to the step 1403 in FIG. 14,various control data for Lo V/T are selected at a step 2001, followed bythe program proceeding to the step 1405 in FIG. 14.

According to a first aspect of the invention, if a failure has occurredin the value timing control means, various control data for Lo V/T areselected, the knocking discrimination levels are set on the basis of theselected control data (step 1405), and it is determined in response tothe output of the knocking sensor 24 whether or not knocking hasoccurred (step 1406).

That is, the control data for Lo V/T are selected irrespective of theactual valve timing, in the event of a failure in the valve timingcontrol means. Therefore, when Lo V/T is correctly selected by the valvetiming control means in a normal condition, the knocking discriminationlevel aLo is obtained to thereby effect normal knocking control, whereasif Hi V/T is wrongly selected due to a failure in the valve timingcontrol means, the knocking discrimination level is obtained bymultiplying the noise level NL_(H) for Hi V/T by the multiplier termG_(L) for Lo V/T and adding thereto the addend term OS_(L) for Lo V/T.As described above with reference to FIGS. 17 and 18, the noise level atHi V/T is lower than that at Lo V/T, and accordingly the multiplier termG_(L) and the addend term OS_(L) for Lo V/T are smaller than those forHi V/T, respectively, so that the knocking discrimination level becomesa considerably smaller value when the valve timing control means isabnormal than when it is normal. Consequently, the answer at the step1406 is liable to become Yes more often when the valve timing controlmeans is abnormal, compared with the case where it is normal, so thatthe ignition timing is retarded in the former case. Thus, when thereexists a failure in the valve timing control means, the ignition timingis retarded at the time of occurrence of knocking at a lower level tothereby eliminate same.

According to a second aspect of the invention, the failsafe operation iscarried out in accordance with a subroutine in FIG. 21. At a step 2001,a predetermined retarding value θ_(RETF/S) for retarding the ignitiontiming is determined from a table shown in FIG. 22, and the valueθ_(RETF/S) is set as the ignition timing correction variable θig_(KN) ata step 2002, followed by termination of the program. Although, as shownin FIG. 22, the predetermined retarding value θ_(RETF/S) increases asthe engine rotational speed Ne increases, the value θ_(RETF/S) mayincrease as the engine load increases.

Therefore, according to the second aspect of the invention, if thereexists a failure in the valve timing control means, the program isterminated immediately after execution of the step 1409' (the failsafeoperation in FIG. 21), as shown in FIG. 14A. That is, the knockingcorrection value θig_(KN) is set to the predetermined retarding valueθ_(RETF/S) to obtain the ignition timing θig and then effect ignition,without using the knocking control means responsive to the knockingsensor output for controlling knocking. By thus inhibiting the operationof the knocking control means, knocking can be prevented irrespective ofthe actual valve timing.

If it is determined at the step 1406 in FIG. 14 that there existsknocking, the ignition timing correction variable θig_(KN) is calculatedin accordance with a subroutine shown in FIG. 15. The variable θig_(KN)is set e.g. to a value of 0 as an initial value in respective firstloops of the subroutine in FIG. 15 and a subroutine in FIG. 16,hereinafter referred to.

At a step 1501, a count value CNO counted by a counter over which noknocking has taken place is set to a value of 0. Then at a step 1502, avalue of the correction variable θig_(KN) is calculated by substractingthe predetermined correction amount Δθ_(RD) for retarding the ignitiontiming from a value of the correction variable θig_(KN) obtained in thelast loop, followed by the program proceeding to a step 1503 where it isdetermined whether or not the value θig_(KN) obtained at the step 1502is larger than the predetermined retarding limit value θig_(KNRD). Thepredetermined retarding limit value θig_(KNRD) is determined dependingon the engine rotational speed Ne and the engine load. If the answer atthe step 1503 is Yes, that is, if θig_(KN) >θig_(KNRD), the correctionvariable θig_(KN) is set to the predetermined retarding limit valueθig_(KNRD) at a step 1504, whereas if the answer is No, the correctionvariable θig_(KN) is set to the value θig_(KN) calculated at the step1502.

On the other hand, when it is determined at the step 1406 that there isno knocking, the ignition timing correction variable θig_(KN) iscalculated in accordance with the subroutine of FIG. 16, in order toreturn or advance the ignition timing which has been retarded inaccordance with the subroutine in FIG. 15.

At a step 1601, a predetermined count value CAV to be counted isretrieved from a map, not shown, depending on the engine rotationalspeed Ne and engine load. The predetermined count value CAV correspondsto a predetermined number or TDC signal pulses over which no knockinghas taken place. At a step 1602, the count value CNO of the counter isincreased by a value of 1, and then it is determined at a step 1603whether or not the value obtained by adding the value of 1 to the countvalue CNO is larger than the predetermined count value CAV retrieved atthe step 1601. If the answer at the step 1603 is Yes, that is, if thecount value CNO has reached the predetermined count value CAV, the countvalue CNO is set to the value of 0 at a step 1604, followed by theprogram proceeding to a step 1605 to calculate the correction variableθig_(KN) by adding the predetermined correction amount Δθ_(AV) foradvancing the ignition timing to the value of the correction variableθig_(KN) obtained in the last loop. If the answer at the step 1603 isNo, that is, if the count value CNO has not reached the predeterminedcount value CAV, the program skips over the steps 1604 and 1605 to astep 1606.

At the step 1606, it is determined whether or not the value of θig_(KN)obtained at the step 1605, or, if the answer at the step 1603 is No, thevalue of θig_(KN) obtained in the last loop is larger than thepredetermined advancing limit value θig_(KNAV). The predeterminedadvancing limit value θig_(KNAV) is determined depending on the enginerotational speed Ne and engine load. If the answer at the step 1606 isYes, that is, if θig_(KN) >θig_(KNAV), the correction variable θig_(KN)is set to the value θig_(KNAV), whereas if the answer is No, thecorrection variable θig_(KN) is set to the value θig_(KN).

The control parameters such as the correction amount Δθ_(RD) forretarding the ignition timing, the correction amount Δθ_(AV) foradvancing the ignition timing, the predetermined retarding limit valueθig_(KNRD), the predetermined advancing limit value θig_(KNAV), and thepredetermined count value CAV are set as follows, depending on the valvetiming:

    ______________________________________                                                     Hi V/T Lo V/T                                                    ______________________________________                                        Δθ.sub.RD                                                                        Large    Small                                                 Δθ.sub.AV                                                                        Large    Small                                                 θig.sub.KNRD                                                                           Large    Small                                                 θig.sub.KNAV                                                                           Large    Small                                                 CAV            Large    Small                                                 ______________________________________                                    

This is, the charging efficiency at Hi V/T is higher than that at Lo V/Tso that the compression ratio at Hi V/T is actually larger than that atLo V/T, whereby knocking is more liable to take place at Hi V/T. Inorder to prevent knocking at Hi V/T, the control parameters are set tolarger values at Hi V/T than at Lo V/T, as shown in the above table.

Although in the embodiment of the invention described above, theignition timing is controlled in order to prevent knocking at the timeof changeover of the valve timing, this is not limitative to theinvention, but fuel supply control to enrich the air-fuel ratio orsupercharging pressure control to reduce the supercharging pressure maybe employed instead of the ignition timing control, in order to preventknocking.

Further, although in the embodiment, the valve timing is changed betweenLo V/T and Hi V/T, the valve timing may be varied in a continuousmanner, whereby the knocking discrimination parameters or knockingcontrol parameters are varied in a continuous manner in accordance withthe continuously varied valve timing.

What is claimed is:
 1. A knocking control system for an internalcombustion engine having inlet valves and exhaust valves, at least oneof said inlet valves and said exhaust valves having valve timing thereofcontrolled by valve timing control means depending on operatingconditions of said engine,the system comprising: valve timing detectingmeans for detecting the valve timing controlled by said valve timingcontrol means; knocking parameter detecting means for detecting adetection parameter indicative of knocking occurring in said engine;knocking discriminating means for determining whether or not knockinghas occurred in said engine on the basis of said detection parameterdetected by said knocking parameter means and at least onediscrimination parameter; knocking control means responsive to an outputfrom said knocking discriminating means indicative of a determinationresult that knocking has occurred, for controlling an operation of saidengine by the use of at least one control parameter, so as to eliminateknocking; parameter value selecting means for selecting a value of atleast one of said at least one discrimination parameter and said atleast one control parameter, which corresponds to the valve timingdetected by said valve timing detecting means; abnormality detectingmeans for detecting abnormality in said valve timing control means; andfailsafe means for holding the valve timing at predetermined valvetiming irrespective of an operating condition in which said engine isoperating, and selecting a valve of at least one of said at least onediscrimination parameter and said at least one control parameter, whichcorrespond to said held predetermined valve timing, when abnormality insaid valve timing control means is detected by said abnormalitydetecting means.
 2. A knocking control system as claimed in claim 1,wherein said detection parameter detected by said knocking parameterdetecting means is a level of vibration of said engine, said vibrationcomprising a knocking component and a noise component, said knockingdiscriminating means setting a knocking discrimination level on thebasis of a level of said noise component and said at least onediscrimination parameter, in a manner deponding upon the valve timingcontrolled by said valve timing control means, insofar as said valvetiming control means normally operates, and determining that knockinghas occurred when said knocking component has a level higher than saidknocking discrimination level.
 3. A knocking control system as claimedin claim 2, wherein said valve timing control means controls the valvetiming to low speed valve timing suitable for a low engine rotationalspeed region and high speed valve timing suitable for a high enginerotational speed region, said failsafe means holding the valve timing atsaid low speed valve timing irrespective of an operating condition inwhich said engine is operating, and selecting a value of said at leastone discrimination parameter corresponding to said held low speed valvetiming, when abnormality is detected in said valve timing control means.4. A knocking control system as claimed in claim 3, wherein said atleast one discrimination parameter comprises first correction values(G_(L), OS_(L)) for said low speed valve timing and second correctionvalues (G_(H), OS_(H)) for said high speed valve timing, said firstcorrection values correcting the level of said noise component by anamount smaller than said second correction values.
 5. A knocking controlsystem as claimed in claim 4, wherein said first correction values andsaid second correction values increase as at least one of the rotationalspeed of said engine and load and said engine increases.
 6. A knockingcontrol system as claimed in any of claims 1-5, wherein said knockingcontrol means corrects ignition timing of said engine in such adirection as to retard same in response to an output from said knockingdiscriminating means indicating that knocking has occurred.
 7. Aknocking control system for an internal combustion engine having inletvalves and exhaust valves, as least one of said inlet valves and saidexhaust valves having valve timing thereof controlled by valve timingcontrol means depending on operating conditions of said engine,thesystem comprising: knocking discriminating means for determining whetheror not knocking has occurred in said engine; knocking control meansresponsive to the valve timing controlled by said valve timing controlmeans and an output from said knocking discriminating means, forcontrolling an operation of said engine so as to eliminate knocking;abnormality detecting means for detecting abnormality in said valvetiming control means; and failsafe means for inhibiting operation ofsaid knocking control means when abnormality in said valve timingcontrol means is detected by said abnormality detecting means.
 8. Aknocking control system as claimed in claim 7, wherein said knockingcontrol means corrects ignition timing of said engine in such adirection as to retard same by a retarding amount corresponding toknocking in response to an output from said knocking discriminatingmeans indicating that knocking has occurred.
 9. A knocking controlsystem as claimed in claim 8, wherein said failsafe means inhibits theoperation of said knocking control means when abnormality in said valvetiming control means is detected by said abnormality detecting means,and corrects the ignition timing of said engine in such a direction asto retard same by a second retarding amount determined in response to atleast one engine operating parameter in place of said first-mentionedretarding amount corresponding to knocking.
 10. A knocking controlsystem as claimed in claim 9, wherein said at least one engine operatingparameter comprises the rotational speed of said engine, said secondretarding amount being set to a larger value as the rotational speed ofsaid engine increases.